(1). Field of the Invention
This invention relates to manufacturing of opto-electronic devices, and in particular angled-facet semiconductor laser packages and sub-assemblies for use in external cavity lasers and amplified spontaneous emission (ASE) sources.
(2). Description of the Related Art
The manufacture of external cavity lasers and ASE sources based on semiconductor laser diodes requires that anti-reflection coatings be applied to the semiconductor laser diode so that the optical reflection from one or more of the laser diode facets is substantially reduced.
By reducing the reflection from one facet, an external cavity semiconductor laser is formed by introducing spectrally controlled reflection from an external element such as a diffraction grating. A laser cavity is formed between the external reflecting element and the back facet of the laser diode. With appropriate design of the cavity, broadly tunable narrow-linewidth lasers can be implemented as described in U.S. Pat. No. 5,050,179.
In the case of an ASE source, anti-reflection coatings are typically applied to both semiconductor laser facets to eliminate the formation of a laser cavity. With appropriate design of the semiconductor laser device, broad spectral bandwidth ASE sources (super-luminescent laser diodes) can be implemented.
It has been demonstrated that orienting the semiconductor laser waveguide at an angle with respect to the laser facet is an effective technique to reduce the effective facet reflectivity without requiring anti-reflection coatings. This is desirable because extremely high-precision anti-reflection coatings with optical power reflectivity of less than 0.1% are required for most external cavity and ASE source applications. This is difficult to achieve in large-scale manufacturing and adds significantly to the cost and reliability of external cavity lasers and ASE sources based on semiconductor laser diodes. Angled-facet semiconductor lasers have been used to implement both external cavity lasers using a curved-waveguide single-angled-facet structure as explained by P. J. S. Heim et al in "Single-facet diode for widely tunable external cavity semiconductor lasers with high spectral purity", Electronics Letters, Jul. 31, 1997, Vol. 33, No. 16 and ASE sources (super-luminescent diodes) using an angled stripe structure as described in U.S. Pat. No. 4,856,014.
One difficulty with angled-facet devices is that the optical beam enters and exits the device at an angle relative to the facet surface normal. In most optical systems it is desirable to define a fixed optical axis that is perpendicular to an established reference plane, i.e. a surface of the laser diode package. The angled-facet device introduces an arbitrary oblique angle that complicates, and thereby discourages, its use in conventional optical systems as exemplified by Heim et al. One application where angled-facet devices have been successfully applied is tilted-stripe angled facet traveling wave semiconductor optical amplifiers, as described by J. V. Collins et al in "Passive alignment of second generation optoelectronic devices", Selected Topics in Quantum Electronics, Vol. 3, No. 6, 1997. However, in these applications custom sub-assemblies are developed to accommodate the angled facet in order to implement a self-contained optical fiber-coupled module. It would be highly desirable to supply the angled-facet semiconductor device so that it is compatible with standard laser diode packages as shown in U.S. Pat. No. 5,262,675 and can therefore be directly incorporated into existing optical systems without having to change the design of the optical system.
A conventional semiconductor laser package is shown in FIG. 1. It comprises a semiconductor laser chip 30 that has been soldered to a sub-mount pedestal 20. The submount pedestal is attached to the header base 10 and electrically connected to contact pin 12. A photodiode 70 mounted on the front surface of the header base 18 detects the optical signal 42 emitted from the back facet 33 of the semiconductor laser. Electrical connections to the semiconductor laser chip 13 and photodiode 11 are provided via bond wires 50 and 51, respectively. The semiconductor laser optical waveguide 31 is oriented perpendicular to the front facet 32 and back facet 33 of the semiconductor laser so that the front optical beam 40 and back optical beam 42 are emitted perpendicular to the respective facets. The semiconductor laser chip is attached to the sub-mount pedestal 20 with front facet 32 parallel to the front edge of the sub-mount pedestal 21, which is also parallel with the front surface of the header base 18, so that both front and back semiconductor laser optical beams propagate parallel to the sub-assembly optical axis 15. The output light beam 40 from the sub-assembly emerges through a glass window 62 that has been anti-reflection coated with films 61 and 63 to reduce optical loss. The window 62 is attached to a cap structure 60 that is welded to the header base 10 in a hermetic sealing process.
The entire package shown in FIG. 1 comprised of the header 10, with mounted semiconductor laser 30, and attached window cap 60 is often called a "TO-can" package. This highly successful semiconductor laser package can be found, for example, in every manufactured compact disk player, laser pointer, and semiconductor laser bar-code scanner. The primary features of this package is that the optical beam is emitted parallel to the optical axis of the package which is in a well-defined direction perpendicular to the plane of the header base 10. The position of the optical beam is axially centered on the header base to facilitate positioning and alignment of the beam. The window cap 60 also provides physical protection to the semiconductor laser and enables the entire assembly to be hermetically sealed.
However, the assembly shown in FIG. 1 has the drawback that the light beam propagates in a direction perpendicular to the window 60, causing light reflecting from the window to couple back into the optical waveguide 31. This is particularly undesirable for external cavity laser and ASE source application because the back-reflections set up a parasitic laser cavity which degrades the performance of the device. To avoid reflection, anti-reflection coatings 61 and 63 are used, but expensive and not efficient. When an angled-facet laser diode is used, it is difficult to have the optical signal emitted from the back facet 33 for the photo-sensor 70 to be parallel to the light emitting from the front window 62. Also, for many external cavity laser applications it is advantageous to include a polarization plate in front of the semiconductor laser to improve laser performance as described by H. Lotem et al in "Tunable external cavity diode laser that incorporates a polarization half-wave plate" Applied Optics, vol. 31, 1992. Even though the laser performance could be substantially improved, it is often difficult or expensive to insert a polarization plate in existing or planned external cavity laser optical systems since it require extra space.